Please open your "Kindle User Guide", before you reply that it is not possible. In the KUG Amazon used GIF89a images for the keyboard symbols, or for example for the pin image symbol, that scales nicely with the font size.

My sw version is: Kindle 3.3 (611 680021)

P.S. As an aside, when I tried to view KUG in the Kindle Previewer, then the software nicely reproduced the behaviour of my Kindle. Then it got autoupdated to V2.4, and now it doesn't reflect the behaviour of my Kindle at all: the GIF images are not scaled according to font size. Bravo, Amazon!

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The solution:

Edit your .gif file with a hexa file editor like Hxd editor. Insert the following bytes before the last byte, 3B:
21 FF 0B 4B 49 4E 44 4C 45 20 20 31 2E 31 01 01 00
Save. You are ready, The .gif image will be scalable on KIndle 3, when you use it in your publication.